Interrupt handling

Abstract: To speed up interrupt processing by interrupt handlers executing with interrupts disabled, one or more processor registers in the computer system are reserved for exclusive use by software executing with interrupt disabled. Interrupt processing code can be written in a high level language. If the code generated by the high level language compiler uses non-reserved registers, these registers are saved by the interrupt handler in the reserved registers before the interrupt processing code is invoked. After execution of the interrupt processing code, the interrupt handler restores the non-reserved registers from the reserved registers. Saving the non-reserved registers in the reserved registers rather than in a memory improves the interrupt processing speed.

Description: A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. The present invention relates to computer systems, and more particularly to interrupt handling in computer systems. A typical computer system includes a processor which executes software stored in a computer memory. When an interrupt occurs, the processor executes software called an interrupt handler. On occurrence of an interrupt, the computer system hardware and/or the interrupt handler typically save the processor registers which can be destroyed by the interrupt handler because these registers may be needed for the interrupted program. General purpose registers, i.e. , registers which can be modified directly by arithmetic and/or logical operations, are saved in the memory. Before returning to the interrupted program, the interrupt handler and/or the system hardware restore the saved registers. The register saving increases the interrupt latency defined as the time elapsed between the moment when the system takes an interrupt and the beginning of the execution of the interrupt processing code. The interrupt processing code is that part of the interrupt handler which is not related to register saving and restoration. To increase the system speed, it is desirable to improve the interrupt latency and also to improve ( i.e. , reduce) the register restoration time at the end of interrupt handling. Improving the interrupt latency and the register restoration time is particularly important for interrupt handlers executed with interrupts disabled because other interrupts may be missed while such an interrupt handler is executing. Thus, it is desirable to provide interrupt handling techniques with reduced interrupt latency and reduced register restoration time. Because saving and restoring general purpose registers involves a memory access, and because a memory access takes a relatively large amount of time, it is particularly desirable to improve the efficiency of general purpose register handling. Further, the interrupt handling techniques should be suitable for writing at least a portion of the interrupt processing code in a high level language since writing code in a high level language facilitates development and maintenance of computer systems. ELECTRONIC DESIGN vol. 40, no. 22, 2 Nov. 1992 CLEVELAND, OH, US. Pages 44-58 compares and contrasts code for fast context switching for reduced instruction set (RISC) and complex instructions set (CISC) processors. It describes interrupt handling code which operates with interrrupts enabled and which saves registers by pushing on to a temporary stack. The present invention provides interrupt handling techniques which, in some embodiments, allow obtaining a short interrupt latency and a short register restoration time. These techniques are suitable in some embodiments for writing an interrupt processing code in a high level language. The above and other advantages are achieved in some embodiments by reserving a number of general purpose registers for code executed with interrupts disabled. Thus, an interrupt handler executing with interrupts disabled may use the reserved general purpose registers without saving them. If an interrupt handler uses any non-reserved general purpose registers, the interrupt handler saves the non-reserved registers in the reserved registers rather than in the memory. Since register access is typically faster than memory access, the interrupt latency is improved. The register restoration time is also improved because restoring the non-reserved registers from the reserved registers is faster than restoring from the memory. The invention is suitable in some embodiments for writing interrupt processing code in a high level language. If the high level language compiler generates code which uses non-reserved registers, the interrupt handler, before invoking the interrupt processing code, saves the non-reserved registers in the reserved registers. Other features and advantages of the invention are described below. The invention is defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a computer system according to the present invention. Fig. 2 is an assembly language macro for installing an interrupt handler according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates a computer system 110 providing a short interrupt latency and a short register restoration time. Processor 114 and memory 120 are connected by interface 124. In some embodiments, interface 124 is a collection of conductors connecting the respective address, data and control ports of the processor and the memory. In other embodiments, interface 124 includes a memory controller, address latches, data buffers, and/or other circuitry as is known in the art. Processor 114 and memory 120 are formed of discrete components or are embedded into an integrated circuit. In some embodiments, processor 114 is a RISC microprocessor of type Am29000 (trademark), Am29050 (trademark), or some other processor of the 29K (trademark) family, which processors are available from Advanced Micro Devices, Inc. of Sunnyvale, California. Memory 120 stores software which includes programs 130 and interrupt handler 140. The software is executed by processor 114. Processor 114 includes general purpose registers 144 and special purpose registers 148. Processor 114 can perform arithmetic and/or logical operations directly on the contents of general purpose registers 144. Special purpose registers 148 provide controls and data for certain processor functions. In a processor of type Am29050 the special purpose registers include, for example, the ALU Status Register which contains information about the outcome of some arithmetic and logical operations. Registers 148 include also the Current Processor Status Register 148.1 which contains control information associated with the currently executed process. See Am29050 Microprocessor User's Manual , Advanced Micro Devices, Inc., 1991. See also RISC Design-Made-Easy SM Application Guide , Advanced Micro Devices, Inc., 1992. Processor 114 does not perform arithmetic and logical operations directly on registers 148. These registers are accessed by data movement only. For example, a special purpose register can be written from a general purpose register 144. In processors of type Am29050, the processor hardware does not save special purpose registers 148 when an interrupt occurs. The hardware, however, freezes the special purpose registers by setting the FZ (Freeze) bit in Current Processor Status Register 148.1. If an interrupt handler uses any special purpose registers, the interrupt handler saves the contents of such registers before resetting the FZ bit. Some interrupt handlers do not use any special purpose registers 148, and these handlers can be executed entirely in Freeze mode (that is, with the FZ bit set). These handlers do not save any special purpose registers improving the interrupt latency and the register restoration time. Many interrupt handlers use one or more general purpose registers 144. If these registers are used by the interrupted program, their contents need to be saved by the interrupt handler. To improve the interrupt latency and the register restoration time, computer system 110 reserves one or more general purpose registers 144.1 for exclusive use by software running with interrupts disabled. This is practical for processors having a large number of general purpose registers, including processors of type Am29050 which have 192 general purpose registers. See Am29050 Microprocessor User's Manual , supra . Many interrupt handlers run with interrupts disabled, and such interrupt handlers can use the reserved registers 144.1 without saving them. Interrupt latency is improved as a result. The register restoration time is reduced to zero if the interrupt handler uses only the reserved registers. In some embodiments using a processor of type Am29050, registers 144.1 are 17 general purpose registers GR64 through GR80. If the interrupt processing code is written in a high level language, for example, in C, the interrupt processing code will use whatever registers are allocated by the high level language compiler. If the compiler allocates non-reserved registers 144.2, then the interrupt handler saves the allocated non-reserved registers in registers 144.1 before invoking the interrupt processing code. The saved registers are restored from registers 144.1 after the interrupt processing code is executed. Saving the registers in the reserved registers 144.1 rather than in the memory improves the interrupt latency and the register restoration time. Fig. 1 includes a block diagram of interrupt handler 140 which saves registers 144.2 in registers 144.1. Fig. 2 illustrates an Am29050 assembly language macro installing the interrupt handler 140. Appendix A illustrates a C language interrupt processing code invoked by interrupt handler 140. At step 140.1, interrupt handler 140 saves registers 144.2 that are used by the interrupt processing code. Registers 144.1 are registers GR64 through GR80 of a processor of type Am29050. Registers 144.2 are Am29050 registers GR1 and GR81 through GR255. Registers GR1 and GR81 through GR95 are reserved for special uses, mostly by the operating system. See RISC Design-Made-Easy , supra , page 1. The system compilers by design generate code which uses only the general purpose registers GR96 through GR255. The compilers start register allocation for the generated code from register GR96 and proceed in the order of register numbers -- GR96, GR97, and so on. At step 140.1, the interrupt handler saves registers GR96 through GR111 in the respective registers GR65 through GR80. The interrupt handler saves in register GR64 the register LR0 which is one of registers GR128 through GR255. The register number of register LR0 is determined by register GR1 as described in Am29050 Microprocessor User's Manual , supra , §7.1.1.3. If the compiler generated code does not use all the registers GR96-111, the interrupt handler in some cases does not save all these registers as explained below. If the compiler generated code uses general purpose registers other than GR96-111, these registers are saved in memory 120. The code for saving the registers in memory is not shown. Before installing the interrupt handler, one determines which general purpose registers are used in the compiler generated code. This determination is made by examining the generated code assembly language listing produced by the compiler. The listing for the interrupt processing code of Appendix A is shown in Appendix B. The Appendix B listing shows only the registers GR98, GR97 and GR96. Once the general purpose registers used by the generated code are determined, an entry point into the interrupt handler is set as follows. Each "add" instruction at step 140.1 (Fig. 2) is a move instruction saving one of registers GR111 through GR97, LR0, GR96 (in that order). The entry point is set so as not to save registers with numbers higher than the largest number of a register used in the generated code. Thus for Appendix A, the entry point is set at the instruction "add gr67,gr98,0" to save only the registers GR98, GR97, GR96 and LR0. The entry point is set by the "nregs" parameter to the macro of Fig. 2. (nregs=3 in Appendix A -- see the last statement of Appendix A.) The macro is then invoked to install the interrupt handler. The last instruction at step 140.1 is "add gr65,gr96,0". While this "add" instruction appears after the instruction "calli" in Fig. 2, this "add" instruction is executed before the completion of "calli". In processors of type Am29050, the instruction following the "calli" instruction is executed before the completion of the "calli" instruction. Once the registers have been saved, the C language interrupt processing code is invoked at step 140.2 by calling the routine identified by the macro parameter "C_handler". The interrupt processing code runs with interrupts disabled. Then, at step 140.3, the saved registers 144.2 are restored from registers 144.1. The first instruction --"jmp"--at step 140.3 transfers control to an "add" instruction in the register restoration code so as to restore only those registers which have been saved at step 140.1. At step 140.4, a return is made from the interrupt handler. The above interrupt processing technique allows writing interrupt processing code in a high level language thereby facilitating the software development and maintenance.

Claims: A method for handling an interrupt in a computer system using an interrupt processing code, the computer system (110) comprising a processor (114) which comprises: a set S2 (144.2) of one or more general purpose registers; the interrupt processing code comprising instructions overwriting one or more general purpose registers of said set S2, characterised in that the processor further comprises a set S1 (144.1) of one or more registers reserved for use by a computer code executed with interrupts disabled; and in that the method comprises the steps of: saving (140.1) one or more general purpose registers which can be overwritten by said interrupt processing code in one or more registers of said set S1; and then invoking (140.2) said interrupt processing code. The method of claim 1 wherein said interrupt processing code is written in a high level language. The method of claim 1 or claim 2 wherein the step of saving saves only those general purpose registers which can be overwritten. The method of any preceding claim further comprising, after execution of said interrupt processing code, the step of restoring (140.3) one or more registers saved by said saving step from one or more registers of said set S1. A computer system (110) comprising: a processor (114) comprising a set S1 (144.1) of one or more registers and also comprising one or more general purpose registers (144.2); a store (120) comprising software which accesses the registers of said set S1 only with interrupts disabled, said store comprising an interrupt handler (140) which comprises: an interrupt processing code comprising one or more instructions overwriting one or more of said general purpose registers; and a code (140.1) for saving the one or more general purpose registers which can be overwritten by said instructions in said one or more registers of said set S1. The computer system of claim 5 wherein the code for saving saves only those general purpose registers which can be overwritten. The computer system of claim 5 or claim 6 wherein said interrupt handler further comprises a code for restoring (140.3), after execution of said interrupt processing code, the saved general purpose registers from the registers of said S1. The computer system of any one of claims 5 to 7, wherein the one or more registers of said set S1 are general purpose registers.

Inventors: MANN DANIEL P, MANN, DANIEL P.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES INC.
Application year: 1994
Main classification: G06F9
IPC codes: G06F9